The present invention relates to a control device and method for a pressure-actuatable bridging coupling of a hydrodynamic torque converter and, more particularly, to a control device in which a pressure regulating valve is connected to a pressure-medium feed system, to a pressure-medium discharge system and to a sensor to avoid the need to increase the pressure level of the hydrodynamic working circuit to engage the bridging coupling.
In known controls for a bridging coupling, the bridging coupling is disengaged by bringing the working pressure of the working-pressure chamber to a higher value than the so-called converter counter pressure which subjects the respective axial piston to pressure in the opposite direction and which exists in the housing interior axially bounded by the radial housing wall and the turbine wheel. For the purpose of engaging the bridging coupling, this type of control device continues to work by relieving the working-pressure chamber and increasing the converter counter pressure. The increase in the converter counter pressure is obtained by increasing the converter inflow pressure and therefore the total pressure of the hydrodynamic working circuit. Since the housing interior communicates with the hydrodynamic working circuit via the gap between the outflow end of the pump wheel and the adjacent inflow end of the turbine wheel, when there is an increase in the pressure level there also necessarily occurs in the working circuit an increased internal housing pressure, in other words, an increased converter counter pressure results and subjects the axial piston to pressure to engage the bridging coupling. The increase in the pressure level of the hydrodynamic working circuit for the purpose of engaging the bridging coupling is a disadvantage because the converter should actually be cutoff; an increased pressure level is not needed at all. Furthermore, there is an increased danger that the converter housing will expand at the high converter speeds occurring in the coupling mode. The known control device takes measures, at relatively low converter speeds, to operate the bridging coupling as a torsion damper with slip during acceleration, but during deceleration to select rigid bridging so as to allow engine braking.
An object of the present invention is to provide a control device and method in which an increase in the pressure level of the hydrodynamic working circuit for the purpose of engaging the bridging coupling is unnecessary and it is possible to affect the working pressure during the engagement of the bridging coupling.
The foregoing object has been achieved by connecting a pressure regulating valve to a pressure-medium feed system, to a pressure-medium discharge system and to a sensor. A reference control pressure is set as a function of converter inflow pressure and converter flow-off pressure analogous to the internal housing pressure of the converter housing. A separating piston is axially displaceable in a bore of the sensor and is connected to a working pressure line so as to be operated by a spring bias and the reference control pressure and also by the working pressure of the working pressure chamber.
In the control device and method according to the present invention, the working pressure acts in the direction of engagement, so that the hydrodynamic working circuit is not subjected to pressure during the actuation of the coupling and therefore no swelling of the converter housing can occur under these circumstances.
In the control device and method according to the invention, the converter housing receiving three bladed wheels and additionally the bridging coupling, together with its actuator, does not have to be equipped with additional passage points for the direct measurement of the converter counter pressure acting counter to the working pressure on the axial piston. A reference control pressure corresponding to the converter counter pressure and composed of the converter inflow pressure and convertor flow-off pressure is set and is activated in a sensor for influencing the behavior of the working pressure during the engagement of the bridging coupling.
To make it possible to obtain a rapid filling of the emptied working-pressure chamber after the initiation of the engagement operation and a subsequent ramp-like behavior of the working pressure during engagement, a coupling control valve is connected to the working pressure line via a throttle and a bypass control valve operating in dependence on the sensor. The bypass control valve is held in a position cutting off the throttle when the working pressure is equal to or lower than the reference control pressure reduced by the amount of tension exerted by a spring.